Extensively altered pore water, with a composition different from that of any previously reported pore water by DSDP/ODP, have been recovered by deep coring of sediments in Izu-Bonin fore-arc sedimentary basin (Leg 126 shipboard scientific party, 1989 ; Fujioka et al., 1989). Upper oligocene vitric and pumiceous sandstone are the dominant products of Izu-Bonin fore-arc basin, where accumulation rates of volcanogenic sediment are extremely high and the sediments are susceptible to alteration. Here we report indications of biogenic alteration. A total of 20 volcanic sandstone samples from Holes 793B were observed by optical microscopy in preparations of polished thin section, transmission electron microscopy, X-ray powder diffraction, Fourier Transform Infrared Spectrometer, and X-ray photoelectron spectrometer.The data show presence of CH, COO, C-0, C-C groups and graphite, suggesting that the green spherical clays (glckuconitic mica clays) in the volcanic sandstone formed not only from
… More hydrothermal alteration products, but also by microbiological interaction between filamentous bacteria and organic materials. The biological materials and debris appear to influence the nature of both the green spherical clays and graphite. Microscopical observation has revealed the presence of bacteria and graphite which can induce the formation of green spherical clays in hydrothermal fluids of porous volcanic sandstone. The sandstone contains numerous hydrothermally altered minerals such as zeolites, anhydrite, smectite, and cristobarite. According to these mineralogical assemblage, the conditions of the products were estimated to be below 200 ﾟC as low-temperature hydrothermal fields near seamounts (Tazaki, 1991 ; Tazaki and Fyfe, 1992).Recently the Joint Global Ocean Flux Study addresses the role of ocean processes in global biogeochemical cycles, with special attention is given to air-sea exchanges of C02. Our observations show that bacteria are abundant in marine hydrothermal systems, and they must contribute to later C02 generation in the altered materials during diagenesis. Less